The present invention is concerned with a hydrogel based pharmaceutical dosage system that provides sustained release of pharmaceuticals without the need to use special coatings or structures that add to the cost of making a sustained release formulation.
When an unmodified hydrogel is used as a pharmaceutical carrier for many diverse types of pharmaceuticals, the in vitro release rates are not zero-order release rates because the initial rate of release of the drug tends to be much higher than the subsequent rate of release of the drug.
In the prior art, hydrogels have been used to prepare sustained release formulations. These formulations have commonly exhibited an "initial burst effect" which causes a non-linear release rate of a drug. In order to avoid the initial burst effect, hydrogels have been used in combination with mechanical devices and polymeric coatings to control the rate of drug release in order to modify the release rate characteristics and provide a substantially zero-order controlled release formulation.
The present invention provides a method for modifying the release characteristics of a hydrogel by adding an effective amount of an ionizable compound to the hydrogel and drug formulation.